1. Field of the Invention
The present invention relates to a magnetic recording medium and a method for manufacturing the same.
2. Description of Related Art
Magnetic recording, media include a magnetic tape, a magnetic disk, a magnetic card and the like. A magnetic tape, which is one of the media, is used for various applications such as audio applications, video applications and computer applications. As for a data backup tape for computers, with an increase in capacity of hard disks to be backed up, magnetic tapes having a recording capacity of several hundred gigabytes (GB) per single tape are commercialized. However, with a further increase in capacity of hard disks, an increase in demand for backup tapes having a large capacity exceeding 1 terabyte (TB) is inevitable. Accordingly, a further increase in capacity of magnetic tapes is crucial.
There are two approaches for increasing the capacity of a magnetic tape: an approach to recording/reproducing apparatuses and an approach to recording media.
The approach to recording/reproducing apparatuses involves, for example, shortening the wavelength of recording signals and narrowing track pitch. This approach reduces the leakage flux from a magnetic tape, so an MR head, which can yield a high output even with a very small flux, is often used as a reproducing head.
The approach to recording medium involves, for example, micronization of a magnetic powder and improvement of magnetic characteristics of a magnetic powder.
Conventionally, as the magnetic powder for forming data backup tape for computers, the same magnetic powders as that used for audio tape or for video tape for household use (e.g., magnetic powders composed of ferromagnetic iron oxide, Co-modified ferromagnetic iron oxide, chromium oxide, etc.) have been used. Currently, the use of an acicular metal powder of ferromagnetic iron having a particle size of about 25 to 65 nm is proposed.
An acicular magnetic powder exhibits magnetic characteristics by its magnetic shape anisotropy. Accordingly, when the particle size is decreased, the coercive force (Hc) decreases, and short wavelength recording characteristics tends to be low. However, a hexagonal Ba-ferrite magnetic powder, an iron nitride magnetic powder, and the like (fine magnetic powders) having a plate or granular shape, which exhibit magnetic characteristics by the crystal magnetic anisotropy even when the particle size is as small as about 5 to 30 nm, are proposed as materials for magnetic recording media.
However, even with use of any of the acicular, granular and plate magnetic powders, if the filling property in a magnetic layer is not increased while reducing the pore size of the magnetic layer, a sufficiently large capacity cannot be achieved. At the same time, in order to allow them to be used in an MR head, which is an approach from recording/reproducing apparatuses as described above, it is necessary to reduce the surface roughness of a magnetic layer to make a smooth surface. To this end, as an approach from production techniques, an improvement of a drying step in which a magnetic coating film is converted into a magnetic layer by removal of solvent is proposed in various documents (see, for example, JP H10-124866A, JP 2004-19958A, JP S63-42030A, JP H4-229415A, etc.).
JP H10-124866A discloses, an apparatus for manufacturing a magnetic recording medium including a plurality of orientation devices equipped with orientation magnets and driers. In this manufacturing apparatus, the temperature and flow rate of the hot air supplied from each drier can be controlled for each orientation device. Also, in this manufacturing apparatus, setting is performed such that the flow rate of the hot air supplied from the plurality of orientation devices increases sequentially along the proceeding direction of a non-magnetic support.
JP 2004-19958A discloses a drying device including not less than two dry zones. In this drying device, a low temperature dry zone is arranged most upstream, and a high temperature dry zone is arranged on a more downstream side than this low temperature dry zone. In the low temperature dry zone, low temperature gas ejection nozzles capable of ejecting a gas having a low temperature and a low humidity are arranged. In the high temperature dry zone, high temperature gas ejection nozzles capable of ejecting a gas having a temperature higher than that of the gas ejected from the low temperature gas ejection nozzles are arranged.
JP S63-42030A discloses an example of a method for manufacturing a magnetic recording medium. According to this manufacturing method, a solvent having a boiling point of not less than 130° C. is contained in a magnetic coating material. Moreover, during a drying process including a constant rate drying stage and a falling rate drying stage, the atmospheric temperature after the falling rate drying stage is set to 40 to 80° C. In JP S63-42030A, the atmospheric temperature of the constant rate drying stage is usually higher than that of the falling rate drying stage. Accordingly, in the method for manufacturing a magnetic recording medium described in JP S63-42030A, there is a moment at which the drying rate (the amount of solvent removed in a unit time) is constant, but constant rate drying is not performed actively. During a constant rate drying stage, the drying rate is constant, whereas in a falling rate drying stage, the drying rate decreases as the amount of the solvent contained in a coating film decreases.
JP H4-229415A discloses a magnetic recording medium and a method for manufacturing the magnetic recording medium. The amount of the solvent remained in the magnetic layer of this magnetic recording medium is 5 mg/m2. JP H4-229415A discloses that the drying rate in the constant rate drying stage of the drying process is set to 1 to 20 kg/m2·hr. However, JP H4-229415A does not disclose that the constant rate drying period is set to 0.2 seconds or more.
As described above, in any of the above-described manufacturing apparatuses and manufacturing methods, the falling rate drying stage is performed without passing through a relatively long constant rate drying stage. This is because if a constant rate drying stage is performed for a longer time, disadvantages will arise such as an increase in size of drying equipment and orientation equipment, and a decrease in production speed.
Moreover, according to the methods described in the above documents, the filling ratio of the magnetic powder in the magnetic layer cannot be increased sufficiently, resulting in insufficient smoothness.
In view of the above, the present invention provides a magnetic recording medium in which the filling ratio of the magnetic powder in the magnetic layer is high and great smoothness is achieved, and a method for manufacturing the magnetic recording medium.